Piezo-electric device and method op producing the same



Dec, 19, 1922.

' A. McL. Nl COLSON.

PIEZO-ELECTRIC DEVICE AND METHOD OF PRODUCING THE SAME.-

FILED APR. 11. 192]. 2 SHEETS-S HEET 1.

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" Dec. 19, 1922.

1,438,965. A. McL. NICOLSON.

PI EZO-ELECTRIC DEVICE AND METHOD OF PRODUCING THE SAME;

FILED APR. II. I92l. 2 SHEETS-SHEET 2.

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Patented Dec. 19, 1922.

UNITED STATES.

PATENT OFFICE.

ALEXANDER MQLEAN' N ICOLSON, OF HILLSDALE, NEW JERSEY, ASSIGNOR TO WEST-ERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y.. A CORPORATION OFNEW YORK.

PIEZO-ELECTRIC DEVICE AND METHOD OF PRODUCING THE SAME.

Application filed April 11, 1921. Serial No. 460,396.

To all whom it may concern:

Be it known that I, ALEXANDER; .MoL; NIcoLsoN, a subject of the King ofGreat Britain, residing at Hillsdale, in the county of Bergen, State ofNew Jersey, have invented certain new and useful Improvements inPiezo-E-lectric Devices and Methods of Producing the Same, of which thefollowing is a full, clear, concise, and exact description.

This invention relates to piezo-electric devices and methods ofproducing the same.

The invention aims to produce crystals having high piezo-electricactivity and to provide a sensitive, efficient, simple and durablepiezo-electric crystal device for translating electrical variations intomechanical vibrations, and vice 'versa, and therefore capable of servingas a telephone transmitting or receiving device for instance.

Other objects of the invention will-be apparent from the detaileddescription hereinafter of the accompanying drawings.

Of the many mineral and organic crystals investigated by H. and P. Curieand others, (see F. Pockels, Winkelmanns Ha'ndb. d. Phys, BD. IV, p.783) the crystal of Rochelle salt (sodium potassium tartrate, N KC H,O4l-l O) was found to have the largest piezo-electric constant,approximately 10 electrostatic units per kg. Apparently no other crystalhas yet been found to approach the piezoelectric activity of fliRochellesalt, particularly if the crystal is 'carefully chosen and speciallyprepared. It

has been found that Rochelle salt is susceptible of greatly increasedpiezoelectric acr tivity. An absolute electric charge of 200electrostatic units'per kg. pressure has been 40 obtained, resulting inpotentials as high as ,500 volts and alternating currents ashigh as20'microamperes. Acoustic tones from a crystal may be heard at adistance of several hundred feet. Briefly the increased efliciency isbrought about by the following conditions:

1. Selection of particular habit of growth.

2. Desiccation.

3. Development of the crystal into a composite polar structure.

crystal growing in accordance These conditions will be considered in deytailin connection with the drawings.

Fig. l of the drawings is a sectional elevation showing a vesselcontaining two crystals growingin accordance with a method comprised inthis invention; Fig. 2 is a plan of the apparatus and crystals shown inFig. 1; F ig. 3 is a perspective view of a crystal grown in the mannerindicated in Figs. l and 2; Fig. 4 is a sectional elevation showing avessel containing a crystal growing in accordance with a second methodcomprised in this invention; Fig. 5, is a plan of the apparatus andcrystal shown in Fig. 4; Fig. 6 is' a perspective view of a crystal rownin the manner indicated by Figs. 4 an 5; Fig. 7 is a sectionalelevationof aIvessel containing a with a third method comprised in thisinvention; Fig. 8 is a plan of the apparatus and crystal shown in Fig.7; Fig. 9 is a perspective view of a crystal grown in the mannerindicated in Figs. 7 and 8; Fig. 10 is a perspective View of apiezo-electric crystal illustrating the hour glass marking appearing incrystals produced in accordance with the methods comprised in thisinvention; Fig. 11 is an elevation, partly in section, of apiezo-electric crystal telephone receiving or transmitting devicecomprised in the invention; and F ig. 12 is a perspective view of thedevice shown in Fig. 11, with the addition of a spirally corrugatedcylindrical diaphragm.

Referring especially to Figures 3, 6, 9 and 1.0 which indicate thegeneral appearance of crystals grown in accordance with this in:vention, the crystal surfaces may be classified into two systems ofsurfaces or zones which are normal to each other. of zones is parallelwith the principal or crystallographic axis 0 and therefore en girdlesthe crystal. ,The other system of zones comprises the two allel with theaand b axes. In accordance with this invention the crystals are rown.yvith thejc and b axes in a horizonta posi-- One system basal planesparzontally on top of the support 19 with thetion as is indicated inFigures '1, 2, 4t, 5, 7 and 8. This growth forms a particular habit,becoming dominant along the c and b axes, while development along the aaxis uperally from a previous cropping, those seeds are selected inwhich growth along the b axisis fully developed (which happens when theseed nucleus grows with its a and b axes horizontal). These seeds aresquare, or nearly so, signifying that the growth along the b axis isabout as great as, or greater than, that along the c axis. The thicknessof the selected seed crystal along its a axis is generally {a b. Thisdimenslon is a function of the density and head of the mother .liquor. 1

In Figs. 1 and 2 the vessel 13 contains crystals-15 and mother liquor17. The crystals rest on cylindrical, upwardly convex supports 19. Theheight of the supports .and the depth of the mother liquor are sorelated that the surface of the mother liquor is at the level of the topof, the crystal','whereby the upward growth of the crystal is prevented.

In growinga crystal, in the manner indicated in Figs. 1 and 2, aseedling 21 to the bottom of which a small quantity of wax or likeadhesive has been applied is placed hori- 0 axis of the seedlingparallel to the surface of the support, the seedling and the motherliquor bein between a temperature of 38 C. and 35 and the concentrationof the mother liquor being such that the density of the solution at 50C. would be about 1.33. The apparatus is then allowed to cool frombetween 387 and 35 -C., to room temperature,

for instance to about-20 C. On thus cooling rapidly the seed-"crystalwill increase in size from a few grams weight to 50 or 500 gramsaccording to the volume and density of liquor used. Growth occursprincipally during the first 10 hours, although'it may be continuedseveral days during the condition of super-saturation if the crystal isallowed to remain in the mother liquor. An average size of crystalweighs 100 grams and its 6 and c axial lengths each approximate mm., theproportions of the crystal being about those indicated in Figure 3. Thecooling of the apparatus to room temperature ordinarily requires fromabout five to ten hours, the time being largely dependent on masses thevolume of mother liquor used. The rate of cooling is fairly rapid atfirst and tapers off as the temperature of the apparatus approaches theroom temperature. The crystal usually grows to the desired size, forinstanceto such a size that it weighs about 100 grams, during thisperiod of from five to ten hours, and may be taken from the motherliquor when it has attained such size.

It appears probable that this rapid growth,

due to the rapid cooling of the mother liquor causes the crystal todevelop internal stresses producing strain region's'symmetrical with theprincipal axis. The crystal thus acquires a composite structure closelyrelated to the surface zones referred to above and to the electric polestobe developed in a manner which will now be set forth. -"At each end ofthe seed nucleus and along its 0 axis there appears a pyramid the baseof which forms a polar terminal. The pyramids are not always verypronounced in the seed nucleus, but are more evident in the growncrystal after the latter has been desiccated as described hereinafter.The

parallelto the b axis and normal to the c axis and to the remainder. ofthe crystal structure, the stratification of which ordinarily isparallel with the 0 axis. The electrical performance 'of the crystalsuggests pyramids in the crystal are indicated at'23 inFigure 10. Theyconsist of stratifications that the crystal molecules throughout thepyramidal regions during growth aresubject ,to forces which turn them,in planes containing the principal axis, through a right angle. This isindicated by the fact that the crystal, after subsequent treatmenttending to render more pronounced these pyramidal terminations, developsits electrical poles (plus and minus corresponding with the conventionalanalogous and antilogous poles) in accordance with the two systems ofzones referred to above. The poles are accordingly orthogonal, that is,at right angles to each other. This signifies that the bases of thepyramids, which occupy almost the entire area of the basal planes of thecrystal, are

electrically plus when the rest of the crystal structure isminus andvice versa. The effect becomes very pronounced when the crysta], issubsequently desiccated in a manner which will now be described.

The desiccating treatment comprises submerging the crystal in 95%alcohol for about twenty-four hours, and alcohol for about six hours,removing the crystal from the alcohol and subjecting it to low ressureand to heat. The alcohol should be rought to the temperature of thecrystal before the crystal is introduced into it, in order that thecrystal may not suffer'stresses due to contact with a liquid of higheror lower temperature. The low premure' used for drying the crystal ispreferably a pressure of about one This treatment of the crystal islargely forremoving waters of inclusion.

The crystal prepared in the manner described above will no longer beclear as when it was taken from the mother liquor, but will be cloudy oropalescent, the surface of the crystal showing irregular white spots andthe crystal exhibiting the pronounced hour-glass marking indicated at 23in Fig. 10. The raw crystal as taken from the mother liquor possesses alarge number of local electric poles which are variable in theirpiezo-electric effect. The desiccated crystal is much stronger in effectand its electric poles are readily found in the case of the compositecrystal to reside as described on the crystal surfaces withcorresponding signs, respectively, in the zones parallel to the c axisand the zones normal to the c axis.

In order to prepare the crystal for piezoelectric use. a tinfoilelectrode is pressed around the girdle pole of the crystal and tinfoilelectrodes are pressed on the basal planes, that is, the planesperpendicular to the c axis. The tinfoil is waxed on the side to beapplied to the crystal. to make it adhere closely to the crystal.Beeswax has been found suitable for this purpose.

Referring to Figure 11 (described in detail later) it will be noted thatboth basal planes 27 and 29 of the crystal 15 are slightlv concave inthe central or polar regions. This feature is obtained in a crystaleither by filing the basal planes of the crystal or by growing thecrystal in a particular manner described hereinafter. The object of thisfeature is to render salient the ends of the basal planes of thecrystal. -As pointed out in the article on the piezo-e-lectric effect inthe composite Rochelle salt crystal, by A. M. Nicolson in theproceedings of the A. I. E. E., November, 1919, these ends of the basalplanes are found to bemechanically sensitive regions of the crystal,just as were the ends of the basal planes of the corresponding crystaldescribed in the U. S. patent application of Nicolson #295,967, filedMay 9, 1919. entitled Piezo-electric transmitter. Rendering the diagonalcorners of the crystal salient is for the purpose of insuring that whenthe crystal is mounted between compressor plates such as 53 and in thegeneral manner indicated in Fig. 11. the pressure brought to bear on thecrystal will be effective on these corners, so that stresses transmittedfrom the plates to the crystal will be efficiently applied to thecrystal.

Obviously, this purpose may also be realized by leaving the basal planesof thecrystal flat and making the compressor plates concave toward thecrystal.

For the purpose of obtaining an efficient piezo-electric action at leasttwo of these four diagonal corners at one crystallographic pole of thecrystal should be perfect and flawless, apart from the compositestructure referred to above.

As pointed out in the Nicolson article re-' ferred to above in the A. I.E. E. roceedings analysis of the direction of applied stresses to thesensitive regions described has shown that with crystals having thecomposite structure referred to above a: given force produces thegreatest piezo-electric effect when the force is applied in such a way.as to twist the crystal about its principal axis, and conversely, anapplied electrical force produces the maximum mechanical response in theform OftWiStlIlg motion, and therefore it is apparent that whetherelectrical or mechanical results are to be produced from the crystal thetorsional effect should be utilized as far as possible, for instance asthe graphophone transmitter described in the Xicolson application#295,967, referred to above utilized the torsional effect in apiezo-electric crystal.

A crystal grown in the manner described above has a reentrant bottomface as shown at 25 in Fig. 3, and due to the prevention of upwardgrowth has but little thickness in the plane through the a and c axes.Due to this reentrant face and this thinness, the crystal is easilydeformable by stresses torsional with respect to the principal axis ofthe crystal. It .appears that the piezo-electric effect of the crystalis thereby greatly increased. The area. of the basal planes or end poles27 and 29 should not be too small. and therefore, the crystal as a wholeshould not be too thin. Therefore. the cylindrical support 19 should notbe too nearly flat.

An alternative method of growing the crystal is shown in Fig. 4, wherea. crystal '31 is shown growing in avessel 33 which forms a mould forthe crystal. The crystal and the vessel 33are placed in the motherliquor in the vessel 35. The bottom of vessel 33 is convexed upward inorder that the lower face of the crystal may be concave for the reasonpointed out above. The depth of the mother liquor is so adjusted thatupward growth of the crystal is limited in order that the crystal may bereadily deformable for the reason pointed out above. The vertical walls37 of the vessel 33 normal to the principal axis of'the crystal. areslightly concave inward except near their ends 39. in order that thebasal planes 41 and 42 of the crystal may be slightly concave for thepurpose pointed out above. The substantially vertical walls 38 of themould or iii vessel 33 flare slightly outwardly from botton to top. tofacilitate the removal of the crystal from the mould.

ln Fig. 7, a crystal 51 is shown growing in a vessel 53 which forms amould tor the crystal since the vessel 53 is similar to the vessel 33.However, the vessel 53 is deeper than the vessel 33. The height of themother liquor in the vessel 53 may be such that even after the crystalhas grown to the desired size the top of the crystal is a considerabledistance below the surface of the liquor, as shown in Fig. 7 or on theother hand the amount of mother liquor in the vessel 53 may be such thatas the crystal grows and the level of the mother liquor theretore falls,the top of the crystal will reach the surface of the liquor so thatiurther'upward development oi" the crystal is prevented. Where theupward growth of the crystal is not prevented, the area of the basalplanes is larger, as indicated in Figs. 7 and 9., but the crystal is notso readily deformable.

A piezo-electric crystal, such as 15 for instance, mounted as shown inFig. 11 between two electrically conducting compressor plates such as 53and 55 with its basal planes electrically connected together to form apole of one sign, and its surfaces normal to the basal planeselectrically connected to form a pole of opposite sign, may be used totranslate mechanical vibrations into electrical vibrations or viceversa, the plates readily conveying jars and vibrations to the crystal.The members 53 and 55 are preferably of aluminum. 'With a crystal havinga chan- ,liO

ncl shaped race parallel to the principal axis single tie-rod 59 lyingwithin the channel formed by the concave -lace may be used to hold thecrystal and the plates 53 and in the desired relation. An insulatinglayer of ruhber tape 36 may be wound upon this tie-rod. The rod mayserve to electrically connect plates 53 and 55. The tie-rod ispreierablyriveted to one or the plates as shown at 60.. The other plate may heslipped on. the tie-rod and held against the crystal with the desireddegree of pressure by means of a nut 32 threaded on the tie-rod. Urthelatter plate may itself be threaded on the tie rod, in which case thenut 52 is not essential but may serve to lock the plate in the positionin which it exerts the desired degree of pressure on the crystal. Aspring washer 3a is preferably interposed between the plate and the nut.The degree oi; static pressure exerted upon the crystal by the platespreterably corresponds toan absolute force of about 15 kg., since, asexplained in the Nicolson application #295,967 referred to above and inthe Nicolson article in the A. l. E. proceedings referred to above, thecomposite crystal is then most. sensitive to variatwn's of pressuresapplied so as to bear on the salient ends of the basal planessimultaneof the plates with the crystal surfaces nor mal thereto itnecessary to prevent slippage between the plates and the crystal due totorsional stresses. If additional security be called for, short pins maybe screwed into the plates to anchor the cement. The pins wouldpreferably be placed one near each end of each of the basal planes ofthe crystal.

A binding screw 61 in one oi theplates may be utilized for connectingone pole or" the receiving or transmitting device .to the circuits inwhich the device is to be used. To electrically connect the girdleelectrode of the crystal with the circuits in which the transmitting orreceiving device is to be used, several layers of tin foil are wrappedaround the girdle electrode as shown at 63, and several turns of wire 65are wrapped around the tin foil 63, several layers or wrappings of tinfoil 67 and a layer of rubber tape 69 being then applied. The tieod 59passes between the crystal and all of these wrappings. The wire 35 ispreferably brought out from the device through an insulating bushing 71in one or" the plates 53 or 55.

As indicated above, the metal plates in Fig. 11 readily convey jars andvibrations.

to the crystal. The crystal and its mounting as shown in Fig. 11therefore constitute an efficient generator or transmitter forconverting the energy of sound waves into the energy of similarelectrical waves. ll this crystal transmitter be laid on a large sheetof paper and receivers be applied to the electrodes, sound produced byrubbing on the paper, or by the ticking of a" watch, will betransmittedto the plates by the paper and can be heard in the receivers.Care should be taken that the tie-rod 60 does not protrude below plate55, since when the transmitter plate 55 rests on a vibrating surface thevibrations are most etficiently, transmitted to the crystal it theportions of the plate adjacent its perimeter are in intimate contactwith the vibrating surface.

The device of Fig. llimay also be used as Ell-ti iao a receiver, sincethe application of electrov ates 53 21116155 in opposite rotational mostefficiently as a translator of electrical into mechanical energy, orvice versa, when the vibrations towhich it is subjected are, as faraspossible, tors'ionalwith'respect to the c 1,4as,eeu

axis of the crystal. Thus, it has been found that the clli'ciency of thedevice shown in Fig. ll may be materially increased by ap-- paper. Thepaper is rolled around the'pe-' riphery of disk 55 and clamped theretoby means of the clamp 77. The cylinder so formed by the paper is thentwisted about its axis and clamped to the periphery of the disk 53 bymeans of the clamp 7%). Spiral corrugations 81 are thus formed in thepaper diaphragm and when the device is used as a transmitter orelectrical generator, mechanical stresses normal to the. surface of thecylinder tending to vary the length of the cylinder produce torsionalstresses in the crystal. When the device is used as a receiver,torsional vibrations electrically produced in the crystal twist thediaphragm, thereby etliciently setting up corresponding air vibrations.

As stated in the Nicolson article in the proceedings of the A. I. E. E.mentioned above, the crystal may be considered as a leaky condenserhaving a shunt resistance in excess of 100 megohms. and a capacityvarying with the size of the crystal and usually of the order of 10"" F.The impedance of the crystal at acoustic frequencies varies from 100,000to 300,000 ohms. If the crystal receiver is used with a microphonetransmitter the supply potential for the crystal should preferably hestepped up to the crystahby means of a transformer. The iuotional1mpedance of the crystal may be found in the same manner as theniotional impedance of an ordinary telephone receiver.

Properly articulated crystals may be grown much larger than is necessaryfor effective operation, but it is difficult to prevent cracks and flawsin drying and heatlng such large crystals. On the other hand, yery smallcrystals are generally lessefl'ectlvely rendered composite on account ofthe relative'absence of the stresses during shorter growing periods.Hence they are usually less efficient when mounted as described, andtherefore it has been stated above that generally a crystal weighingabout 100 gr. 1s a desirable size. However, 'it is at times de-' sirableto use small crystals, for'instance 5 or 6 mm. long and 2 mm. wide, aswhere the space available for theoperating location of the crystal isvery restricted.

Handling crystals grown In accordance with this invention sometimescauses the crystals to crack, especially if the crystals come intocontact with the hands before the crystals have been desiccated asdescribed above. v This handling may be largely avoided by growing thecrystals in a vessel such as :33 or 53 for instance, pouring off anyexcess mother liquor when the crystal has grown to the desired size, andallowing the crystal to remain in the vessel during desiccation. Thismethod also has the advantage that shrinkage of the crystal duringdesiccation facilitates the removal of the crystal from the mould.

The expressions cylindrical concave and cylindrical reentrant and thelike as used herein to describe crystal faces, are intended to apply toprismatic or other cylindrical hollowed out surfaces having a rightsection of a general arch or C shape, and the arch or C simulated may beas nearly a closed figure as desired. The expression cylindrical convexis used in a similarly broad sense. \Vhere a crystal has a cylindricalup 'ardly convex lower face of which the right section is nearly aclosed figure the crystal may be removed from its mould by sliding thecrystal parallel to its 0 axis, the mould having no end walls. a crystalmay be such that its right section presents a plurality of sharplyreentraut cusps superposed on the general line of reentrancy. for thepurpose of making the crystal the more readily deformable by stressestorsional with respect to the c axis. Such a crystal face would. ofcourse. be obtained by having a complementary face on the mould orsupport used in growing the crystal.

This application is a continuation of my applications Serial No.226,343. filed April 2, 1018. Method of making piezo electricalcrystals; Serial No. 227,802 filed April 10, 1918, Piezophony; andSerial X0. 295,907, filed May S), 1019, Piezo-electrical transmitter, asregards common subject matter.

\Vhat is claimed is:

1. A piezo-electric crystal of sodium potassium tartrate, said crystalhaving. a principal axis and having a cylindrical concave face parallelto said axis.

2. A piezo-electric crystal having a principal axis, said crystal havinga channel shaped face parallel to said axis.

3. A piezo-electric crystal having a principal axis and having areentrant cylindrical face parallel to said axis.

4. A piezo-electric crystal having a principal axis, every cross-sectionof said crystal normal to said axis having a reentrant side.

5. A piezo-electric crystal of sodium p0- tassium tartratehaving aprincipal axis, and having a face normal to said axis, one side of saidface being reentrant.

6. The method of growing a piezo-electric crystal which comprisessupporting said crystal. in concentrated mother liquor, on the upwardlyconvex bottom of a vessel of such size that the grown crystal contactscrystal to the action of concentrated mother liquor in a Vessel of suchsize that the grown crystal contacts with two or more opposite verticalwalls ofsaid vessel.

8. The method of growing piezoelectric crystal .which'comprisessubjecting said crystal to the action of concentrated mother liquor ina. vessel of such size as to form n mould for the grown crystal.

9. The methodof growing e. piezoeleo trio crystal which comprisessubjecting the crystal to the action of concentrated mother liquor andso adjusting the level of the surface of the mother liquor that saidsurface is at least as low as the top of the crystal while the crystalis growing.

10. The method of growing a piezotrio crystal with opposed concave polewhich comprises subjecting the crystal to the action of concentratedmother liquor a vessel having opposed inwardly convex walls until thecrystal moulds itself to the shape of said wells.

11. The method of grow-inpa piezo-electrio crystal which comprisessupporting said crystal on a convex surface in a con centroted solutionof mother liquor.

12. The method of growing a piezo-electrio crystal which comprisessupporting said crystal on a cylindrical convex surface in aconcentrated solution of mother liquor.

13. The method of growing a piezo-electric crystal of sodium potassiumturtrafe which comprises subjecting the crystal to the action of motherliquor of a temperature of between 38 C. and 35 C. and of a density ofabout 1.33 at 50 (1, and lower-' ing the temperature of the motherliquor about 30 C. in less than ten hours.

14. The method of growing a piezo-elec-- trio crystal which comprisessupporting the crystal on a cylindrical upwardly convex surface inmother liquor and increasing the concentration of the mother liquor atarate such that the grown crystal possesses a pronounced compositestructure.

15. The method'of growing a piezo-electrio crystal which comprisessupporting said.

and of a density of about 1.33 at 50 C., and

lowering the temperature of said mother liquor about 30 C. in from fiveto ten hours.

.17. The method of preparing a piezo-elecincense tric crystal whichcomprises soaking said crystal in alcohol and subjecting said crystal toa pressure less than un atmosphere.

18. The method of preparing a. piezo-elec trio crystal which comprisessoaking said crystal in alcohol and subjecting said crystal to apressure loss than an atmosphere and to heat.

l9. The method of preparing a piezo-electrio crystel which comprisessupporting s. crystal of sodium potassium tart-rate on s cylindricalconvex surface in mother liquor of e temperature of between 38 and C.and of a specific gravity of about 1.33 at 50 C, lowering thetemperhture of said mother liquor about 30 C. in from five. to tenhours, removing said crystal from the mother liquor and soaking it inalcohol, removing said crystal from said alcohol and subjecting saidcrystal to at pressure less than an atmosphere and to heat.

20. The method of growing a piezc-electrio crystal which comprisessupporting said crystal. in concentrated mother liquor, on the upwardlyconvex bottom of a vessel of such size that the vertical walls of thegrown crystal contact with the vertical walls of the vessel.

21. The method of growing a piczo-electric crystal which comprisessubjecting a part only of said crystal to the action of super--saturated mother liquor.

22. The method of growing a piezo-electrio crystal which comprisessubjecting said crystal to the action of supersaturated mother liquorand maintaining a part of said crystal at least as high as the surfaceof said mother liquor while the crystal is growing.

23. A piezoelectric crystal having a, principal axis and. having apronounced hour glass marking, said crystal having a channel shaped faceparallel to said axis.

24. A piezoelectric crystal of sodium potassium tartrate, said crystalhaving a principal axis, said crystal comprising pyramids consisting ofstratifications normal to said axis and to the remainder of the crystalstructure, and said crystal having a channel shaped face parallel tosaid axis.

25. A piezo-electric crystal having an arch shape.

26. A piezo-electric crystal having a. principal axis, said crystalhaving a shape such as to renderthe crystal readily deformable bystresses torsional with respect to said axis.

27. The method of growing a. piezo-electrio crystal of a predeterminedshape which comprises subjecting said crystal to the action ofconcentrated mother liquor and limiting the growth of the crystal by amould.

28. The method of prepa ing a crystal which comprises growing it n 2.mol and desiccating it before removal from the'mold.

29. The method of preparing a. piezo-electrio crystal which comprisesgrowing the crystal in a mold and desiccating it in the mold bysoalringit in alcohol and drying it internally.

30. The method of growing a pie-zo-electric crystal from a seedlingwhich comprises supporting: said seedling in a substantially horizontalposition on a rigid extended surface in concentrated mother liquor at atem- ALEXANDER McLEAN NICOLSON.

